Philips Magnetoresistive Sensor manual Flipping coil Sensor KMZ10A1

Philips Semiconductors

Magnetoresistive sensors for

magnetic field measurement

General

The optimal method of compensating for temperature dependent sensitivity differences in MR measurements of weak fields uses electro-magnetic feedback. As can be seen from the sensor characteristics in Figs 31 and 32, sensor output is completely independent of temperature changes at the point where no external field is applied (the null-point). By using an electro-magnetic feedback set-up, it is possible to ensure the sensor is always operated at this point.

To achieve this, a second compensation coil is wrapped around the sensor perpendicular to the flipping coil, so that the magnetic field produced by this coil is in the same plane as the field being measured.

Should the measured magnetic field vary, the sensor’s output voltage will change, but the change will be different at different ambient temperatures. This voltage change is converted into a current by an integral controller and supplied to the compensation coil, which then itself produces a magnetic field proportional to the output voltage change caused by the change in measured field.

The magnetic field produced by the compensation coil is in the opposite direction to the measured field, so when it is added to the measured field, it compensates exactly for the change in the output signal, regardless of its actual, temperature-dependent value. This principle is called current compensation and because the sensor is always used at its ‘zero’ point, compensation current is independent of the actual sensitivity of the sensor or sensitivity drift with temperature.

Information on the measured magnetic signal is effectively given by the current fed to the compensating coil. If the field factor of the compensation coil is known, this simplifies calculation of the compensating field from the compensating current and therefore the calculation of the measured magnetic field. If this field factor is not precisely known, then the resistor performing the current/voltage conversion must be trimmed. Figure 34 shows a block diagram of a compensated sensor set-up including the flipping circuit.

compensation coil

flipping field

earth's field

compensation field

MLC757

flipping coil

sensor KMZ10A1

Fig.33 Magnetic field directions and the flipping and compensation coils.

2000 Sep 06 28